The present invention relates to display apparatuses each having a power supply device and power supply circuits for supplying power to the display apparatuses and more particular, to a display apparatus using a liquid crystal display of a thin film transistor (TFT) type, a liquid crystal display using low temperature polysilicon, organic electronic luminescence (EL), or plasma and to a driving circuit for displaying.
JP-A-10-301087 discloses that a common electrode driving means supplies an A.C. driving voltage to a common electrode as a common driving voltage to A.C. drive the common electrode, a non-selecting period voltage generator generates a non-selecting period voltage in phase with and having the same amplitude as the A.C. driving voltage, and the non-selecting period voltage generator is connected in series with the common driving means to current-amplify and output a voltage dropped by a predetermined level from an A.C. driving voltage as an output of the common driving means. And a common driver includes an operational amplifier to which an A.C. signal is applied, and a buffer circuit having NPN and PNP transistors; while a gate-off voltage generator includes a level shift circuit having a resistor and a Zener diode and a buffer circuit having an operational amplifier and NPN and PNP transistors. Also disclosed in the above literature is that the gate-off voltage generator shifts an output voltage of the common driver at the level shift circuit and current-amplifies the shifted voltage at the buffer circuit.
Also disclosed in U.S. Pat. No. 5,877,736 is that a gate-on voltage generating circuit includes a level shift circuit having a constant current source and a Zener diode and a buffer circuit having an operational amplifier and NPN and PNP transistors, the level shift circuit shifts an output voltage of a common driver, the buffer circuit amplifies the shifted voltage, a gate-off voltage generating circuit includes a level shift circuit having a constant current source and a Zener diode and a buffer circuit having an operational amplifier and NPN and PNP transistors, the level shift circuit shifts the output voltage of the common driver, and the buffer circuit amplifies the shifted voltage. Further disclosed in the above publication is that the waveform of a common voltage Vcom, the on-level waveform of a gate voltage, and the off-level waveform of the gate voltage are different in D.C. level but have the same shape, and thus that the waveform of the common voltage Vcom, on-level and off-level waveforms of the gate voltage can be obtained by generating one of these waveforms and by level shifting the generated waveform to form the other two waveforms. Also disclosed in the Publication is that a variable resistor is used to change the amplitude of the common voltage of an A.C. driving waveform generated by a common voltage generator. Additionally disclosed in the Publication is that a semi-fixed resistor is used to adjust the D.C. level of the common signal voltage.
In the conventional power supply device, however, a current constantly flows from the common voltage through the Zener diode, thus resulting in a high power consumption. Further, in order to change of the voltage amplitude or level of the common voltage and gate-off voltage according to the TFT liquid crystal panel used, it is required to change the value of each resistor or to replace parts such as the Zener diode. In addition, an increased number of parts are required, which is disadvantageous from the cost viewpoint.